"Materialism" is used to refer to people who value possessions--as in "Bill Gates is a materialist." But when we say that Karl Marx was a materialist, we don't mean that he hankered after possessions. And when we say that Marx was not an idealist, we aren't saying he didn't have ideas about how to change the world.

In the history of the philosophy, idealism and materialism have very different meanings than their popular usage. They represent the two main divergent ways of looking at the world we live in.

For the idealist, the mind--or the spirit, in the form of God--is the origin of all material things. The ancient Greek idealist philosopher Plato, for example, argued that the world and the things in it were determined by universal, logical categories. Therefore, every specific tree was a copy derived from the universal category "tree."

Plato--and all other idealists--separated the mind from matter, and argued the former ruled over the latter. The materialist view is exactly the opposite.

For the materialist, all of reality is based on matter, including the human brain which is itself a result of the organization of matter in a particular way. In this view, the abstract idea of "tree" was developed by humans from their experience of actual trees. "It is not consciousness that determines being," wrote Marx, putting it another way, "but social being that determines consciousness."

Probably the most popular form of idealism is "free will"--the idea that individuals can do anything they set their mind too. For example, the view that "you can beat poverty if you really try hard" implicitly accepts the idea of free will. Poverty, in this view, is not a social phenomenon caused by, for example, a plant closing or a chronic illness in the family. Rather, poverty is some kind of personal choice.

The most famous example of this kind of thinking came from former President Ronald Reagan, who once argued that people were "homeless by choice." Ironically, no one has ever argued that multi-billion dollar companies go "bankrupt by choice," Yet every year, in spite of the wills of individual businessmen involved, thousands go bankrupt.

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MARX AND Engels ridiculed the view that ideas determine reality. "Once upon a time, a valiant fellow had the idea that men were drowned in water only because they were possessed with the idea of gravity," they wrote. "If they wee to get this notion out of their heads...they would be sublimely proof against any danger from water."

But by rejecting "free will," Marx didn't embrace "determinism"--the idea that human beings are slaves to the blind forces of history. "The materialist doctrine," wrote Marx, " that men are products of circumstances and upbringing, and that, therefore, changed men are products of other circumstances and changed upbringing, forgets that it is men who change circumstances."

For Marx, people "make their own history, but they do not make it just as they please; they do not make it under circumstances chosen by themselves, but under circumstances directly encountered, given and transmitted from the past."

Human behavior is first shaped by our physical makeup. We must labor cooperatively in order to eat, drink and find shelter. At any given stage in human development, the level of production--and the social relations based on that level of production--shape our limits and possibilities.

"People cannot be liberated," wrote Marx and Engels, "as long as they are unable to obtain food and drink, housing and clothing in adequate quality and quantity. 'Liberation' is a historical and not a mental act and it is brought about by historical conditions."

Ideas can and do shape history--but only if those ideas are embraced by millions and only if the social and material conditions for their realization exist.